1. Field of the Invention
The present invention is broadly concerned with expandable sealant and baffle compositions for sealing hollow structural members of vehicles, and methods for making and using such compositions. The compositions are prepared by forming an expandable mixture including a first thermoplastic resin (preferably an SBS block co-polymer) and an epoxy resin (preferably a bisphenol A-based liquid epoxy resin). The compositions preferably also include a second thermoplastic resin (preferably a polystyrene) different from the first thermoplastic resin, and a compound selected from the group consisting of pigments, blowing agents, catalysts, curing agents, reinforcers, and mixtures thereof. The preferred reinforcers are hydrated amorphous silica and glass microspheres. The compositions of the invention are injection moldable and can be formed into freestanding, self-sustaining parts. Alternately, the compositions of the invention can be supported on lattice-type nylon supports. Upon heating of the compositions to temperatures of at least about 300xc2x0 F., the compositions greatly expand to form lightweight products having high compressive strengths.
2. Description of the Prior Art
During the fabrication of automobiles, trucks, and similar over-the-road vehicles, many body components present structural members having cavities that require sealing to prevent the entrance of moisture and contaminants which can cause corrosion of the body parts. It is also desirable to greatly strengthen the members while maintaining their light weight. It is also necessary to stabilize these members in order to attenuate noise that would otherwise be transmitted along the length or passage of the cavity. Many of these cavities are irregular in shape or narrow in size, thus making them difficult to properly seal and baffle.
Many attempts have been made to seal these cavities, spraying sealants into the cavity, introducing foam products into the cavity, and using of fiberglass matting and the like. These methods each have their drawbacks. For example, foaming in place presents a problem in that it is difficult to control where the foam travels upon its introduction into the cavity. Furthermore, it is often necessary to introduce an excess amount of foam into the cavity in order to ensure that the cavity is sufficiently sealed. Finally, foams will generally not adhere to the interior surfaces of the cavity walls if those surfaces contain even a small amount of oil.
Self-sustaining foam products, either with or without a non-foam support structure, have been introduced into structural member cavities in an attempt to seal the cavities. However, these methods generally result in the addition of excess weight to the structural member which is undesirable in most instances. Attempts have been made to utilize foam products which are lighter in weight or which do not use a support structure. However, these attempts have generally resulted in products which lack the increased compressive strength necessary to adequately reinforce the structural member.
U.S. Pat. No. 5,755,486 to Wycech is directed towards a structural reinforcement member which includes a thermally expandable resin-based material. The structural member is heated so as to expand the resin-based material, thus locking the reinforcement member in place. However, the compositions disclosed in the ""486 patent do not possess sufficient expansion capabilities. As a result, more of the composition is required to adequately fill and seal a particular cavity, thus resulting in added weight to the car and added expense to the automotive manufacturer. Furthermore, the composition disclosed in the ""486 patent makes use of fumed silica which leads to a product having a high viscosity and therefore unsuitable for injection molding of the composition.
U.S. Pat. Nos. 5,373,027 and 5,266,133 to Hanley et al. are respectively directed towards expansible sealant and baffle components for sealing and providing an acoustic baffle for cavities in vehicle bodies and methods of making such components. The components expand upon heating, thus sealing the cavity in which they are placed. The components are formed of a composition which includes an ethylene-xcex1,xcex2 ethylenically unsaturated carboxylic acid copolymer, a blowing agent, a tackifier, and optionally an additive polymer and a cross-linking agent. While the compositions of the ""027 and ""133 patents possess sufficient expansion capabilities, they are not strong enough to make suitable reinforcement members.
U.S. Pat. No. 5,506,025 to Otto et al. is concerned with an expandable baffle apparatus for sealing an automobile cavity. The apparatus of the ""025 patent includes a piece of heat expandable sealing material formed in a shape corresponding to the shape of the cross-section of the cavity to be sealed. The sealing material is supported by an open lattice support element formed of material such as nylon. While the apparatus of the ""025 patent is suitable as a cavity sealant and baffle apparatus, it does not provide sufficient reinforcement to the structural member forming the cavity in which the apparatus is placed.
There is a need for an injection moldable, expandable, lightweight composition which acts as both a sealant to prevent the entrance of undesirable components into vehicle cavities and a baffle to diminish or prevent noises which normally travel along the length of the cavity. Furthermore, this composition should have a high compressive strength so that it reinforces the structural members in which it is used.
The instant invention overcomes these problems by providing an expandable sealant and baffle compositions comprising mixtures of thermoplastic resin(s) and an epoxy resin which are injection moldable and lightweight, and which have a high compressive strengths.
In more detail, the compositions of the invention include a first thermoplastic resin, usually selected from the group consisting of the polystyrenes, rubbers (preferably solid rubbers), and mixtures thereof. It is preferred that the first thermoplastic resin be a solid rubber or mixtures of solid rubbers. Preferred solid rubbers include styrene-butadiene rubber (such as SBR 1009(copyright)), nitrile-butadiene rubber (such as Nipol 1411(copyright)), thermoplastic elastomers including SBS block co-polymers (such as Fina Clear 530(copyright)), and mixtures thereof. (As used herein, xe2x80x9crubberxe2x80x9d is intended to include all synthetic rubbers as well as elastomers.). If the rubber used is an SBS block co-polymer, it is preferred that the SBS block co-polymer have a molecular weight of from about 100,000-150,000, and preferably from about 110,000-135,000. When a styrene-butadiene rubber is used as the rubber, the ratio of butadiene to styrene is preferably from about 32:68 to about 18:82, and more preferably from about 27:73 to about 23:77. If the first thermoplastic resin is a rubber, then the rubber is preferably present in the composition at a level from about 20-30% by weight, and more preferably from about 23-28% by weight, based upon the total weight of the composition taken as 100% by weight.
If the first thermoplastic resin is a polystyrene, then the polystyrene should be present in the composition at a level of from about 5-20% by weight, and preferably at a level of from about 10-15% by weight, based upon the total weight of the composition taken as 100% by weight. It is preferable that the polystyrene have a molecular weight of from about 150,000-320,000, and more preferably from about 200,000-270,000. Two preferred polystyrenes are sold under the trade names Fina Crystal 500(copyright) and Fina Crystal 535(copyright).
The compositions further include an epoxy resin, preferably a liquid epoxy resin such as a bisphenol A-based liquid epoxy resin. The epoxy resin should be present in the composition at a level of from about 30-45% by weight, and preferably from about 35-40% by weight, based upon the total weight of the composition taken as 100% by weight. A preferred solid epoxy resin is available under the trade name Epon 1001 F(copyright). Two preferred liquid epoxy resins are sold under the trade names Araldite 6010(copyright) and Epon 71(copyright).
In a particularly preferred embodiment, the compositions of the inventions further include a second thermoplastic resin different from the first thermoplastic resin. The second thermoplastic resin is likewise preferably selected from the group consisting of the polystyrenes, rubbers (preferably solid rubbers or thermoplastic elastomers), and mixtures thereof It is preferred that the first thermoplastic resin be a solid rubber and mixtures of solid rubbers while the second thermoplastic resin is preferably a polystyrene, with the individual concentrations of solid rubber and polystyrene being those described above. The total weight of both the first thermoplastic resin and the second thermoplastic resin is preferably from about 25-50% by weight, and more preferably from about 33-43% by weight, based upon the total weight of the composition taken as 100% by weight.
In another embodiment, the compositions of the invention further include a third thermoplastic resin in addition to and different from the first and second thermoplastic resins. The third thermoplastic resin is preferably selected from the group consisting of polystyrenes, rubbers (preferably solid rubbers or thermoplastic elastomers), and mixtures thereof. In compositions according to the invention which include a third thermoplastic resin, the third resin should be present in an amount of from about 1-10% by weight, and preferably from about 3-7% by weight, based upon the total weight of the composition taken as 100% by weight. A preferred third thermoplastic resin is a solid rubber such as nitrile-butadiene rubber (NBR). When NBR is used as the third thermoplastic resin, the ratio of acrylonitrile to butadiene is preferably from about 42:58 to about 35:65, and more preferably about 38:62.
The compositions of the invention should also include a compound selected from the group consisting of pigments, blowing agents, catalysts, curing agents, reinforcers, and mixtures thereof. Any pigments, blowing agents, catalysts, and curing agents known in the art are suitable for use in the instant invention. Examples of some of those compounds and their preferred concentrations in the compositions of the invention are set forth in Table 1.
The preferred reinforcers are selected from the group consisting of hydrated amorphous silica, glass microspheres, and mixtures thereof Preferably the compositions hereof include from about 1-10% by weight hydrated amorphous silica and from about 10-20% glass microspheres. Utilizing both of these reinforcers results in a composition having a very high compressive strength. Also, hydrated amorphous silica is important for providing a composition that has high expansion capabilities as well as a viscosity (at 110xc2x0 C.) of less than about 1500 P, and preferably less than about 1250 P, so that the composition can readily be injection molded.
One of the most preferred embodiments of the compositions in accordance with the instant invention includes the following: from about 20-30% by weight SBS block co-polymer; from about 5-20% by weight polystyrene, from about 30-45% by weight bisphenol A-based liquid epoxy resin; from about 0.5-5% by weight carbon black; from about 1-10% by weight hydrated amorphous silica; from about 10-20% by weight glass microspheres; from about 0.5-5% by weight blowing agent; from about 0.3-5% by weight catalysts; and from about 1-5% by weight curing agent, with all percents by weight being based upon the total weight of the composition taken as 100% by weight.
The compositions of the invention are formed by mixing the first thermoplastic resin (preferably solid rubber) with a small portion (about {fraction (1/40)}th of the total amount) of the epoxy resin in a heated mixer until the temperature of the mixer reaches from about 240-260xc2x0 F. (the temperature of the mixture within the mixer is at least about 175xc2x0 F.) and the mixture is substantially homogeneous, at which time all of the second thermoplastic resin (preferably a polystyrene) is added to the mixer and mixing is continued. After the second thermoplastic resin is substantially mixed with the first thermoplastic resin/epoxy resin mixture, the remainder of the epoxy resin is slowly added to the mixer, stopping and starting the mixer as necessary, with the ingredients being thoroughly mixed to obtain a substantially homogeneous mixture. The resulting xe2x80x9cpremixxe2x80x9d can then be used immediately to form the final composition or, alternately, can be formed into a thick sheet (approximately 20 mm) via a two-roll mill, cut into pieces, and stored for later use.
To form the final composition from the premix, the desired amount of premix is placed in a heated mixer (set at a temperature of about 250xc2x0 F.) and mixing is commenced. While mixing, the desired pigments are added to the mixer and mixing is stopped once a homogeneous mixture is obtained within the mixer. While mixing is stopped, one of the desired reinforcers is added to the mixer and mixing is resumed and continued until the mixture is homogeneous. This step can be repeated, adding additional reinforcers as desired.
The temperature of the mixer is then set to a temperature below 160xc2x0 F. When the temperature of the mixer drops below 160xc2x0 F., the desired blowing agents, curing agents, and/or catalysts are added and mixing is resumed and continued only until the mixture is homogeneous. The resulting mixture is then preferably extruded into strands (at an extruder temperature of 170-180xc2x0 F. and screw rotation speeds of about 400 rpm) and cut into pellets. Alternately, the desired blowing agents, curing agents, and/or catalysts can be dry blended with the cut pellets rather than being added to the mixture prior to extrusion. The pellets are then injection molded into the desired shape at a temperature of about 180-200xc2x0 F. using any injection molding techniques and equipment known in the art. Those skilled in the art will appreciate that the desired shape of the molded composition will depend upon the volume of the cavity to be sealed and baffled, as well as the configuration of that cavity thus ensuring that the composition fully expands into a sealing relationship with the structural sidewalls. The molded product that is formed is a physical mixture, and no chemical reactions have occurred prior to the heating of the product within the vehicle cavity to a temperature of at least about 300xc2x0 F.
While the sealing and baffling compositions of the invention can be manufactured as formed, freestanding parts, in other embodiments the compositions can be supported on a support element comprising an open lattice formed in the same shape as of the composition, thus forming a sealant and baffle apparatus. Preferred lattice support elements are formed of nylon, and particularly glass reinforced nylon. Use of open lattice supports allows the composition to expand through the openings of the lattice upon heating of the composition to a temperature of at least about 300xc2x0 F. This structure is superior to the structure in U.S. Pat. No. 5,755486 to Wycech as the use of a lattice support element adds minimal weight to the apparatus.
The expanded composition then adheres to adjacent walls of the cavity in which the apparatus is placed. Preferred apparatuses comprise a plurality of U-shaped expandable composition structures spaced apart from one another and in general alignment, having the lattice support element attached to the three outer surfaces of the U-shaped structure. This embodiment provides for contact by the composition with a greater surface area of the cavity walls than is provided by U.S. Pat. No. 5,506,025 to Otto et al., thus providing superior reinforcement of structural member forming the cavity.
In use, the compositions of the invention are formed into the U-shaped structure described above, or are shaped (in either free standing form or some type of lattice-supported form) to conform to the cross-sectional shape of (although slightly smaller than) the cavity in which the composition or apparatus is to be used. The formed composition or apparatus is then placed within the hollow channel or cavity and heat is applied so that the composition expands as the epoxy resin is crosslinked and the expanded composition adheres to the sidewalls of the cavity resulting in a channel or cavity that is substantially blocked by the expanded composition. Optionally, some type of fastening device or mechanism can be utilized to secure the composition within the channel prior to heat expansion.
It will be appreciated that the compositions of the invention can be used in virtually any area of the automotive industry, including body shops, paint shops, and automobile manufacturing facilities. A particular advantage of the compositions and apparatuses of the invention is that they can be placed within the desired channel prior to conveyance of the vehicle body through a bake oven where the temperatures are sufficiently high to expand the composition.
When the sealants and baffle compositions of the invention are subjected to a temperature of at least about 300xc2x0 F., the percent expansion of the composition will be at least about 95%, preferably at least about 125%, and more preferably at least about 150%, wherein the percent expansion (as used herein) is defined as:
100xc3x97{[(the specific gravity of the composition before heating)xe2x88x92(the specific gravity of the composition after heating)]/(the specific gravity of the composition after heating)}.
The expanded compositions have a compressive strength (using a sample having a diameter of 2 inches and a length of 4 inches and a compression rate of 0.5 inches/minute) of at least about 1200 psi, preferably at least about 1400 psi, and more preferably at least about 1600 psi. Prior to expansion, the compositions have a specific gravity (with reference to water) of at least about 0.90, while the specific gravity (with reference to water) of the expanded compositions is less than about 0.47, preferably less than about 0.37, and more preferably less than about 0.32. The compositions of the invention have a ratio of compressive strength:specific gravity after bake of at least about 2500:1, preferably at least about 3000:1, and more preferably at least about 3600:1.